Other d20 systems

Pathfinder, published by Paizo, is an updated D&D game with a huge amount of additional free material. Pathfinder is very popular among people who don't like D&D 4.

Mutants and Masterminds 3rd Edition modifes the d20 rules to let you play a superhero! This is a really fun and flexible system, allowing you to play pretty much anything you can imagine.

One Shots

I've been building up a nice collection of one-shot games. These systems can be picked up quickly, even by people who aren't familiar with tabletop roleplaying, and they're a lot of fun.

Everyone is John

John is a incompetent and insane. The players are voices in John's head, trying to get him to do what they want. This is a competitive RPG, and takes about an hour to play.Everyone is John.Character Sheet.

All Outa Bubblegum

A very simple RPG with just one stat - bubblegum. You start with eight sticks of gum. To do something mundane, you have to roll lower than your bubblegum on a ten sided die. To do something “kick-ass,” you have to roll higher. Anyone can play this game, and you can even have small children and adults in the same game without having any less fun. I've played with a five year old, and with a room full of people. The game usually takes less than an hour. All Outta Bubblegum

Fiasco

The king of one-shots, Fiasco is a game where you create a Coen-brothers style movie. In case you're not familiar with the type, things tend to go from bad to worse, and your goal could be to make your character's life as horrible as possible. It's played in one of many “playsets,” which are guidelines as to the setting and relationships your characters have. The game set-up is great. You go around in a circle selecting relationships, locations and objects that define what kind of story you're going to create. Fiasco claims that it takes around 45 minutes per player. In my experience, a four-player game takes 3-4 hours. The PDF only costs $12. It's absolutely worth it. Fiasco.

Other Systems

Ars Magica, a system with an extremely complex and flexible magic system, where almost everyone plays a mage. They've released their 4th edition rules to the public domain.

Open Legend RPG, a classless, genre-agnostic system in which you can build your character however you want. Instead of picking class levels, you choose feats to define your character. It bears some resemblance to Mutants and Masterminds.